1. Field of the Invention
The present invention relates to radar systems; and more particularly to synthetic aperture radar antenna systems.
2. Description of the Prior Art
Synthetic aperture radar is based upon the synthesis of an effectively long antenna array that is created by the motion of the radar platform in a relatively straight path at a constant velocity. As the aircraft carrying the radar platform moves along its flight path, a single antenna of relatively small dimension transmits repetitive pulses. The reflected pulses or echoes are sensed and stored in a memory device until a predetermined number of echoes have been received to achieve the desired resolution. The signals stored in such memory device are somewhat similar to the signals that would have been received by a conventional linear array of antenna elements arranged along the flight path. Such systems are particularly advantageous in mapping ground areas from a flight platform. Depending upon the particular application, the antenna element which may be conveniently located in the nose of the aircraft may be pointed in the direction of flight to illuminate a section of ground area directly ahead of the moving aircraft (0.degree.), are directed to a position orthogonal to the flight path to illuminate an area to the side of the aircraft (90.degree.), are directed to a position to illuminate an area between the dead ahead and side looking positions, which is sometimes referred to as the squint mode of operation (0.degree.-90.degree.).
As the aircraft travels, the area that is effectively mapped takes the form of a path. The width of this path, which is referred to as a range swath determines the limits of the ground area coverage. The actual width of such path depends on both the angle of the antenna in elevation, referred to as the depression angle, and the angle of the antenna in azimuth (0.degree.-90.degree.) relative to the direction of aircraft travel. At large depression angles, and small angles in azimuth, the range swath coverage becomes quite narrow.
For some applications, the radar antenna may be mechanically scanned in azimuth to scan the ground area from the dead ahead position through the squint mode to the side looking or orthogonal position as the aircraft is travelling along its flight path. In addition to scanning in azimuth, the depression angle may be varied to select the distance or range of the area being mapped from the aircraft. Such angle, of course, determines the deviation in range of the ground area from the actual altitude of the aircraft. As such scanning occurs, the range swath coverage decreases as the angle in azimuth changes from 90.degree. to the dead ahead position.
The limits of such range swath are determined by the line of constant doppler frequency shift or isodop line. Mapping, of course, occurs along the boresight of the antenna, which is referred to as the isogain line. Thus to obtain an image without distortion, the echoes of a predetermined doppler frequency shift are filtered prior to utilization.
Heretofore, in order to maintain an optimum range swath coverage for a system that operates in a squint mode or scans through such squint mode, the reflected signals were processed in a manner which is sometimes referred to as range dependent processing. Such processing involves continually shifting the reference frequency as the isogain line or antenna boresight relative to the direction of the aircraft shifts; or in other words, a different frequency is utilized depending upon the actual range.